The present invention relates to a reflective liquid crystal display device used for displaying images with reflection of read light beams modulated in accordance with video signals.
There are strong demands for projection-type display apparatus for displaying images on a large screen, such as, apparatus for outdoor public use or those for use in airport control towers, high-precision display apparatus for high vision and projectors.
The projection-type display apparatus is classified into transmission and reflective types, both using a liquid crystal display device. In operation, a read light beam incident into a liquid crystal display device is modulated per pixel in accordance with a video signal, thus converted into a light beam to be projected onto a screen.
Liquid crystal display devices are equipped with an active-matrix substrate aligned on which are switching transistors such as thin-film transistors and pixel electrodes to which voltages are supplied while controlled by the switching transistors. Formed over the active-matrix substrate is a common electrode coated with a light-transmissive substrate (such as a glass substrate). A liquid crystal layer is provided between the active-matrix substrate and the common electrode.
A voltage across the common electrode and each pixel electrode is varied in accordance with a video signal to control orientation of a liquid crystal filled in the liquid crystal layer for modulation of a read light beam.
The liquid crystal display device is also classified into transmission and reflective types.
Transmission-type liquid crystal display devices are equipped with liquid-crystal drive circuitry and wirings in a liquid crystal panel with a width of about 10 μm around pixel electrodes.
This configuration causes a low ratio (aperture) of a pixel area to the total displaying area in the liquid crystal panel. The aperture is more or less 60% even for a transmission-type liquid crystal display device having a highest aperture at present.
Display apparatus such as liquid crystal projectors equipped with the transmission-type liquid crystal display device cannot display images of high intensity because of decrease in aperture due to increase in the number of pixels, thus increase in pixel density, or resolution.
Accordingly, developed and put in practical use recently, instead of transmission-type liquid crystal display devices, are reflective liquid crystal display devices that give high intensity and resolution.
Discussed next are problems caused for known reflective liquid crystal display devices.
Illustrated in FIG. 1 is a cross section of a known reflective liquid crystal display device for each pixel.
Provided on a semiconductor substrate 10 (a P-type silicon substrate) are pixel switching transistor Tr and a capacitor C. The transistor Tr and the capacitor C are electrically isolated by an SiO2 field oxide film 12.
The pixel switching transistor Tr is formed on an N-type well 14. It is an MOSFET constituted by a drain 16 and a source 18, each a highly-dense impurity layer, and a gate electrode 20 situated therebetween via a gate oxide film.
The capacitor C is constituted by a lower electrode (highly-dense impurity layer) 22 and an upper electrode 24 formed over the lower electrode 22 via an insulating film, for storing charges.
Formed over the pixel switching transistor Tr and the capacitor C is a first SiO2interlayer insulating layer 26, patterned on which is an Al wiring layer 28. Formed on the wiring layer 28 is a second SiO2 interlayer insulating layer 30.
Patterned on the second interlayer insulating layer 30 is an Al light shielding layer 32 for light shielding so that a reading light beam is hardly be incident below the shielding layer 32. Formed on the light shielding layer 32 is a third Sio2 interlayer insulating layer 34.
Moreover, formed on the third interlayer insulating layer 34 is a quadrangular Al pixel electrode 4 that is connected to the source 18 of the pixel switching transistor Tr and the upper electrode 24 of the capacitor C, via the light shielding layer 32.
Multiple pixel electrodes 4 are arranged into a matrix over a liquid crystal panel, with a gap 36 between two adjacent pixel electrodes, thus constituting an active matrix substrate.
Provided as facing the multiple pixel electrodes 4 is a transparent common electrode 38 with a light-transmissive (glass-like) substrate 40 formed thereon.
Formed between the multiple pixel electrodes 4 and the transparent common electrode 38 is a liquid crystal layer LC filled with a liquid crystal.
The common electrode 38 is provided as covering over multiple pixels Px. In addition, alignment films (not shown) are formed on the pixel electrodes 4 and the common electrode 38.
The width of each gap 36 between two adjacent pixel electrodes 4, the area without serving light modulation in this type of reflective liquid crystal display device, is about in the range from 0.5 to 0.7 μm. Therefore, reflective liquid crystal display devices having a pixel-electrode pitch of, for example, 14 μm can have aperture in the range from 90 to 93%.
In operation, a reading light beam LT is incident via the light-transmissive substrate 40, as indicated by dot lines in FIG. 1.
It is inevitable that some light components of the light beam LT are incident into the active-matrix substrate as intruding beams LTi via the gaps 36.
Each intruding beams LTi propagates between the pixel electrode 4 and the light-shielding layer 32 and also the shielding layer 32 and the wiring layer 28 while reflected therebetween, as indicated by dot lines.
The intruding beam LTi is finally incident into the drain 16 and/or the source 18 that constitute a PN-junction photo diode. This generates photo carriers to cause a leak current, thus resulting in variation in voltage at the pixel electrode 4, which is a cause of flickering or burn-in.
Such light intrusion could be prevented by a long optical path of each intruding beam LTi with a large pixel electrode 4. Nevertheless, it goes against the trend of pixel miniaturization, and hence cannot be employed.
In order to solve such a problem, for example, Japanese Unexamined Patent Publication No. 2000-193994 discloses an anti-reflection (reflection protective) layer 42 formed on the light-shielding layer 32 before the third interlayer insulating layer 34 formed thereon, as shown in FIG. 1, to attenuate the intruding beams LTi.
The anti-reflection layer 42 is made of a single layer of titanium nitride (TiN) or a double layer of silicon nitride (SiN) and TiN, an SiN film being formed on a TiN film.
Reflective liquid crystal display devices with a single liquid crystal panel use a read light beam within a visible-light having wavelengths of 4000 to 7000 Å. The titanium nitride of the anti-reflection layer 42 can be adjusted as exhibiting a low reflectivity against some wavelengths, but not all wavelengths in the visible-light range, thus reflection blocking being not enough. This is the same for the TiN/SiN anti-reflection layer.
Reflection of intruding light beams may be blocked on each panel in reflective liquid crystal display devices with three liquid crystal panels of red, blue and green. For instance, a anti-reflection 42 used in a liquid crystal panel for red can be adjusted as having a thickness to exhibit a low reflectivity against light of wavelength in the range from 6000 to 7000 Å for red. Nevertheless, this results in difference in thickness for anti-reflection layers in the liquid crystal panels for red, blue and green. In other words, common liquid crystal panels cannot be used for red, blue and green, which causes low productivity.